Something that can never possibly exist due to the deep-rooted feud between ninjas and pirates. And also because ninjas and pirates are so played out already that if someone were to create a pirate ninja, it would be some kind of horrible clichéd supermonster that would not only knock you down with its peg-leg but would also give you a roundhouse kick to the face if you crossed its path.

Poor Johnny didn't even have a chance to defend himself when he accidentally stumbled upon a pirate ninja. But luckily since pirate ninjas can't exist, Johnny was thrown into an alternate parallel universe before the ninja pirate was able to attack.

The unholy union between a pirate and a ninja. Pirates and ninjas are widely known to be mortal enemies and are locked in eternal oppisition to each other.

Pirate-ninjas rarely exist in nature because they have extremely short half-lives; upon their creation they rapidly decay in a violent burst of short-wave radiation, heat, shurikens, and parrot feathers.